The present invention relates to a thin film transistor (TFT) which has a non-crystal (amorphous) or polycrystal semiconductor thin film.
A conventional thin film transistor (TFT) using an amorphous silicon (a-Si) thin film has a lateral structure, whose cross-sectional construction is as shown in FIG. 1a or FIG. 1b.
In FIG. 1a, a gate electrode 4 covered by a gate insulator film 6 is selectively formed on an insulating substrate 1 such as glass, quartz, ceramics, or SiO.sub.2 -coated Si or metal, and further a high resistivity a-Si film 5 is deposited as a channel region on the gate insulator film 6. The main electrodes comprising source electrode 3 and drain electrode 2 are formed on the surface of the a-Si film 5. Further, an insulator film 7 such as an oxide film is frequently used to cover the a-Si film 5 for surface passivation.
The drain and source main electrodes 2 and 3 are composed of a metal film such as Al, Mg, Pt and Mn or metal silicide and an impurity-doped a-Si film.
In FIG. 1b, a gate electrode 4 is formed on the gate insulator film 6 deposited over the high resistivity a-Si film 5. The drain and source wiring metals 12 and 13 are respectively connected to the drain and source electrodes 2 and 3 comprised of a metal or semiconductor thin film. The drain and source electrodes 2 and 3 are formed under the a-Si film 5 and on the insulating substrate 1.
The conventional TFT can be applied to a low price integrated circuit and a large area TFT array (for example a liquid crystal display panel) since a low price glass is used as the substrate 1.
However, the carrier mobility of a-Si is generally less than 1/100 that of single crystal Si, and the a-Si TFT cannot operate at high speed and is therefore used in limited application.
Furthermore, it is so difficult during the TFT manufacturing to introduce a self-alignment technique using ion-implanation (which is conventionally used in manufacturing a single crystal MOS-IC), that a short-length channel TFT is difficult to achieve and this technique cannot be used to improve operation speed.
Moreover, since the conductivity of the a-Si is generally changed by light irradiation, it is necessary for the TFT to have a light-shielding layer when employed in a liquid crystal display panel which results in an increase in production processing and cost.